An investigation into the characteristics of switching converters with chaotic carrier frequency modulation

Abstract

This thesis reports an evaluation of the input current spectral characteristics of switching converters with chaotic carrier frequency modulation scheme (CCFMS). By incorporating a Chua’s circuit into the pulsewidth modulator for driving the switches, three modulation schemes, including the standard pulsewidth modulation scheme (SPWMS), periodic carrier frequency modulation scheme, and CCFMS, can be realized with the Chua’s circuit in equilibrium, limit cycle, and chaos, respectively. Bifurcations and chaos have been observed and analyzed for various kinds of power electronics circuits. Theoretical predictions are verified with experimental measurements. The Chua’s circuit was chosen because it can be controlled by simply varying the resistance of a resistor in order for the circuit to get into different operating states, namely from equilibrium, to period-1 limit cycle, period-2 limit cycle, period-4 limit cycle, spiral chaos and double-scroll chaos. By incorporating the Chua’s circuit signal under different states to dither the switching frequency of the power converters under constant duty-cycle control, the spectral performance of the power converter output voltage can be examined for these operating states. It is found that discrete voltage harmonics of the power converter voltage can only be spread over a reasonable frequency range when the Chua’s circuit gets into either spiral chaos or double scroll chaos. The period-1, -2 and -4 limit cycle signals are not “chaotic” enough to spread the harmonic power effectively. A concept of “degree of chaos” (ℜ) has been employed to quantify the amount of “chaos” introduced to the PWM frequency signal. The degree of chaos is related to the range of frequency deviation from the nominal switching frequency of the power converter. It is found that ℜ=0.35 is sufficient to spread the harmonic power. In particular, this study emphases the suitability and practicality of the chaos implementation of the Chua’s circuit for large-scale production of power converter and control. It has been found that the Chua’s circuit can be adopted into existing PWM integrated circuit control chip within a tolerance (2%) of commercial-grade passive components. Thus, existing PWM integrated circuit can easily be modified to incorporate chaotic switching function for reducing electromagnetic interference (EMI) emission. The study concludes that chaotic switching is an effective method for upgrading existing and well-established PWM control design for meeting international EMC regulations. No redesign of power converter components is needed. The input current and output voltage power spectral densities under different operating regions have been studied theoretically and experimentally. By controlling the dynamics of the Chua’s circuit, it has been shown that CCFMS hybridizes the features of periodic carrier frequency modulation scheme (PCFMS) and random carrier frequency modulation scheme (RCFMS). Experimental results have confirmed that CCFMS is an alternative way for reducing the amplitudes of switching harmonics. Most importantly, CCFMS introduces less low-frequency harmonics at the output than RCFMS. Index Term – DC/DC conversion, power electronics, pulsewidth modulation, random switching techniques, switching circuits, chaotic switching techniques.